Journal: The journal of physical chemistry letters
In nanostructured thin films, photogenerated charge carriers can access the surface more easily than in dense films and thus react more readily. However, the high surface area of these films has also been associated with enhanced recombination losses via surface states. We herein use transient absorption spectroscopy to compare the ultrafast charge carrier kinetics in dense and nanostructured TiO2 films for its two most widely used polymorphs: anatase and rutile. We find that nanostructuring does not enhance recombination rates on ultrafast timescales, indicating that surface state mediated recombination is not a key loss pathway for either TiO2 polymorph. Rutile shows faster, and less intensity-dependent recombination than anatase, which we assign to its higher doping density. For both polymorphs, we conclude that bulk rather than surface recombination is the primary determinant of charge carrier lifetime.
The deposition of particles on a surface by an evaporating sessile droplet is important for phenomena as diverse as printing, thin-film deposition, and self-assembly. The shape of the final deposit depends on the flows within the droplet during evaporation. These flows are typically determined at the onset of the process by the intrinsic physical, chemical, and geometrical properties of the droplet and its environment. Here, we demonstrate deterministic emergence and real-time control of Marangoni flows within the evaporating droplet by an external point source of vapor. By varying the source location, we can modulate these flows in space and time to pattern colloids on surfaces in a controllable manner.
It is known in thin-film deposition that the density of nucleated clusters N varies with the deposition rate F as a power law, N ∼ F (α). The exponent α is a function of the critical nucleus size i in a way that changes with the aggregation limiting process. We extend here the derivation of the analytical capture-zone distribution function P β(s) = a ß ·s (β) ·exp(-b β s (2)) of Pimpinelli and Einstein to generic aggregation-limiting processes. We show that the parameter β is generally related to the critical nucleus size i and to the exponent α by the equality α·β = i, in the case of compact islands. This remarkable result allows one to measure i with no a priori knowledge of the actual aggregation mechanism. We apply this equality to measuring the critical nucleus size for pentacene deposition on mica. This system shows a crossover from diffusion-limited to attachment-limited aggregation with increasing deposition rates.
Chalcogenide perovskites with optimal bandgap and desirable light absorption are promising for photovoltaic devices, whereas the absence of ferroelectricity limits their potential in application. Based on first-principles calculations, we reveal the underlying mechanism of the paraelectric nature of Ba3Zr2S7 observed in experiments and demonstrate a general rule for the appearance of ferroelectricity in chalcogenide perovskites with Ruddlesden-Popper (RP) A3B2X7 structures. Group theoretical analysis shows that tolerance factor is the primary factor that dominates the ferroelectricity. Both Ba3Zr2S7 and Ba3Hf2S7 with large tolerance factors are paraelectric due to the suppression of in-phase rotation that is indispensable to hybrid improper ferroelectricity. In contrast, Ca3Zr2S7, Ca3Hf2S7, Ca3Zr2Se7 and Ca3Hf2S7 with small tolerance factors exhibit in-phase rotation and can be stable in the ferroelectric Cmc21 ground state with non-trivial polarization. These findings not only provide useful guidance to engineering ferroelectricity in RP chalcogenide perovskites but also suggest potential ferroelectric semiconductors for photovoltaic applications.
Hybrid perovskites have emerged over the past five years as absorber layers for novel high-efficiency low-cost solar cells combining the advantages of organic and inorganic semiconductors. Unfortunately, electrical transport in these materials is still poorly understood. Employing the linear response approach of Density Functional Theory, we reveal strong anharmonic effects and a double-well phonon instability at the center of the Brillouin zone for both cubic and orthorhombic phases of inorganic CsPbI3. Previously reported soft phonon modes are stabilized at the actual lower-symmetry equilibrium structure, which occurs in a very flat energy landscape, highlighting the strong competition between the different phases of CsPbI3. Factoring these low-energy phonons into electron-phonon interactions and band gap calculations could help better understand the electrical transport properties in these materials. Furthermore, the perovskite oscillations through the corresponding energy barrier could explain the underlying ferroelectricity and the dynamical Rashba effect predicted in halide perovskites for photovoltaics.
Despite many recent developments in designing and screening catalysts for improved performance, transition-metal oxides continue to prove challenging due to the myriad inherent complexities of the systems and the possible morphologies that it can exhibit. Herein we propose a structural descriptor, the adjusted coordination number (ACN), which can generalize the reactivity of the oxygen sites over the many possible surface facets and defects of a transition-metal oxide. We demonstrate the strong correlation of this geometric descriptor with the electronic and energetic properties of the active sites on five facets of four transition-metal oxides. We then use the structural descriptor to predict C-H activation energies, to show the great potential of using ACN for the prediction of catalytic performance. This study presents a first look into quantifying the relation between active site structure and reactivity of transition-metal-oxide catalysts.
We show that a common Li-O2 battery cathode binder, poly(vinylidene fluoride) (PVDF), degrades in the presence of reduced oxygen species during Li-O2 discharge when adventitious impurities are present. This degradation process forms products that exhibit Raman shifts (∼1133 and 1525 cm(-1)) nearly identical to those reported to belong to lithium superoxide (LiO2), complicating the identification of LiO2 in Li-O2 batteries. We show that these peaks are not observed when characterizing extracted discharged cathodes that employ poly(tetrafluoroethylene) (PTFE) as a binder, even when used to bind iridium-decorated reduced graphene oxide (Ir-rGO)-based cathodes similar to those that reportedly stabilize bulk LiO2 formation. We confirm that for all extracted discharged cathodes on which the 1133 and 1525 cm(-1) Raman shifts are observed, only a 2.0 e(-)/O2 process is identified during the discharge, and lithium peroxide (Li2O2) is predominantly formed (along with typical parasitic side product formation). Our results strongly suggest that bulk, stable LiO2 formation via the 1 e(-)/O2 process is not an active discharge reaction in Li-O2 batteries.
The development of colloidal quantum dot (QD) lasers is blocked by Auger recombination (AR). Here, phase-pure wurtzite CdSe/CdS core/shell QDs with controlled shell thickness are reported, which possess nearly defect-free core/shell interfaces. Benefiting from increased volume, electron-hole partial spatial separation, and nearly defect-free alloyed interface, this series of QDs exhibit a greater than 3 orders of magnitude decrease in AR rates with increasing shell thickness. Consequently, the amplified spontaneous emission threshold of the QDs with an 11 monolayer CdS shell is found to reach a minimum of 16 μJ cm(-2). A record long lifetime (>1000 ps) and extraordinarily large bandwidth (>170 nm) of optical gain are observed by employing ultrafast transient absorption spectroscopy. We leverage the low-threshold gain of the QDs to fabricate microlasers that display single-mode operation and an ultralow threshold of ∼2 μJ cm(-2). Our results represent a valuable step toward practical QD lasers.
Machine learning has been recently used for novel perovskite designs, owing to the availability of large amount of perovskite formability data. Trustworthy results should be based on the valid and reliable data that can reveal the nature of materials as much as possible. In this study, a procedure has been developed to identify the formability of perovskites for all the compounds with the stoichiometry of ABX3 and (A'A'‘)(B'B’‘)X6, that exist in experiments and are stored in the database of Materials Projects. Our results have enriched data of perovskite formability in a large extent and corrected the possible errors of previous data in ABO3 compounds. Furthermore, machine learning with multiple models approach have identified the A2B'B’‘O6 compounds that have suspicious formability results in current experimental data. Therefore, further experimental validation experiments are called for. This work paves a way for cleaning perovskite formability data for reliable machine learning work in future.
Thermally Activated Delayed Fluorescence (TADF) offers the premise for all-organic light emitting diodes with quantum efficiencies competing those of transition metal-based phosphorescent devices. While computational efforts have so far largely focused on gas-phase calculations of singlet and triplet excitation energies, the design of TADF materials requires multiple methodological developments targeting among others a quantitative description of electronic excitation energetics, fully accounting for environmental electrostatics and molecular conformational effects, the accurate assessment of the quantum-mechanical interactions that trigger the elementary electronic processes involved in TADF, as well as a robust picture for the dynamics of these fundamental processes. In this perspective, we describe some recent progress along those lines and highlight the main challenges ahead for modeling, which we hope will be useful to the whole TADF community.